It is well known that sheet steels are usually produced from either rimmed steel ingots or SK steel ingots i.e., special killed steel, killed (deoxidized) with aluminum. Rimmed steel is used in applications where surface quality is the most important requirement and little or no drawability is necessary, whereas SK steel must be used where deep drawability is essential.
More recently, a so-called "rim-stabilized" steel has been developed which incorporates the desired features of both rimmed and SK steels. That is, a rim-stabilized steel ingot has a clean, good surface quality rim approaching that of a conventional rimmed steel and a low porosity core which, when rolled to sheet product, will yield good deep drawing characteristics approaching those of SK steel.
Rim-stabilized steels are presently produced by casting a rimming type steel (i.e., non-deoxidized) into an ingot mold and allowing the steel to rim for a predetermined time thereby forming a good surface quality rim. After rimming, aluminum is added to the unsolidified steel in the ingot mold to stop the rimming action and produce a low porosity SK steel within the rimmed shell.
Although rim-stabilized steels do indeed fulfill a long-felt need for a sheet steel having good surface qualities as well as deep drawability, so many problems are encountered in producing ingots thereof that the steel's properties are not as good as could be hoped for. For example, the time available for adding, melting and distributing the aluminum in an already cast ingot is quite short when considering the rather large amount of aluminum that must be added, e.g., about 2 lb. per ton. Most frequently, therefore, the aluminum is not uniformly distributed within the molten portion of the steel, especially in the lower portion thereof. This results, of course, in non-uniform deep drawing qualities.
Another problem encountered in producing rim-stabilized steel results from the inability to produce a rim which is thick enough to allow removal of all surface imperfections without exposing the non-metallic inclusions in the SK steel therebeneath. Generally, the rim is so thin that only a fast hot-scarfing operation on the rolling mill and a minimal amount of hand grinding is permitted.
Still another problem results from the practice of interrupted teeming to allow the limited rimming action. Inherent in this interruption, usually about 2 minutes, is the build-up of iron oxide scum on the exposed upper surface of the metal which increases with increased rimming time. When aluminum pellets are subsequently added on top of this surface scum, an excessive amount of refractory alumina is formed. Much of this alumina may become entrapped in the steel upon solidification. This alumina problem usually becomes even more aggravated because some of the added aluminum may not be quickly melted nor easily driven below the meniscus of the molten metal. Hence, some of the aluminum may remain on the surface of the melt to be oxidized by air, thereby producing additional quantities of the troublesome alumina. It follows, therefore, that because of the excessive amount of alumina formed, the efficiency of the operation suffers, necessitating the addition of a substantially greater amount of aluminum that is actually necessary to suitably deoxidize the cast steel.
U.S. Pat. Nos. 3,754,591 and 3,865,643 disclose an improved process for producing a rim-stabilized steel ingot which overcomes many of the above problems. The crux of the disclosed process resides in the use of molten aluminum to kill or deoxidize the molten steel core. Specifically, the process involves teeming a non-deoxidized rimming-type steel into an ingot mold to a level about 80 to 95% full. Teeming is then interrupted to allow a rimming action in the mold to progress for a period of from 1/2 to 15 minutes. Thereafter, teeming is again continued until the mold is full. During this final teeming step following the rimming action, sufficient molten aluminum is injected into the teem stream to kill or semi-kill the molten core. Because the aluminum is already molten when it is added to the teem stream, it is more uniformly distributed throughout the core, and with a minimum formation of surface alumina.
Although the above patented process has achieved a considerable degree of commercial success, by virtue of its superior product, the process itself, however, presents a noted disadvantage in that it requires the use of specialized equipment and a trained operator. Specifically, the specialized equipment must include a furnace or crucible to melt and contain the aluminum, a pump to deliver a predetermined quantity of molten aluminum through a refractory-lined delivery pipe and, of course, an energy source to melt the aluminum. All such pumps in commercial use today are fixed-volume, pressure pumps so that it is not possible to accurately vary the amount of molten aluminum added without going to a different pump size. In addition, the equipment must be mobile so that it can follow the ladle during the teeming of a drag of ingot molds. Because of the reaction nature of molten aluminum, the equipment is somewhat difficult to maintain in a good operable condition. The safe and effective operation of the equipment requires the services of a trained operator.